There is a known technique in which a trocar, which is used in laparoscopic surgery and is placed in an opening formed in the skin to allow an endoscope or treatment instrument to be inserted into the body, is provided with a depth-of-insertion detecting unit that detects the depth of insertion of an insertion section of the endoscope or treatment instrument (see, for example, Patent Literature PTL 1). This technique uses the depth-of-insertion detecting unit to detect the depth of insertion of the insertion section by measuring the amount of rotation of a roller in contact with the outer surface of the insertion section.